Remember the ATF says NO MORE THAN 20 POUNDS of smokeless powder at any location/home/shop.

Incorrect.http://www.saami.org/specifications_...ess_Powder.pdf
11-3.7 Smokeless propellants intended for personal use in quantities
not exceeding 20 lb. (9.1 kg) shall be permitted to be stored in original
containers in residences. Quantities exceeding 20 lb. (9.1 kg), but not
exceeding 50 lb. (22.7 kg), shall be permitted to be stored in residences
where kept in a wooden box or cabinet having walls of at least
1 in. (25.4 mm) nominal thickness.

Buy a cheapo 'put-it-together-yourself' wooden cabinet from a big box store and line it with enough plywood to bring the minimum thickness up to 1" and you'll be fine up to 50lbs.
Some localities do have orndinances against ordnances that are more restrictive however.